1. Field of the Invention
This invention relates to an X-ray generating method and an X-ray generating apparatus.
2. Description of the Related Art
In order to generate high intensity X-rays, it is required to irradiate high density electron beam onto a target. It is difficult, however, to generate a minute focal point onto the target from the high density electron beam because of the large repulsive forces of the electrons of the high density electron beam. In order to mitigate such a problem as not generating the minute focal point, it is proposed to enhance the accelerating voltage of the electrons, but in this case, the electrons are introduced deeply into the target so that the X-rays generated from the deep portions of the target is absorbed into the target and thus, the generating efficiency of the intended X-rays is lowered. When the accelerating voltage is enhanced, the cost of the X-ray generating apparatus may be increased because the X-ray generating apparatus must be insulated entirely.
In Reference 1, referring to the first paragraph in “Summary of the Invention” at col. 1, the invention is directed at providing an X-ray source of type described wherein several different sizes of the X-ray focal spot are possible at low cost. Concretely, referring to FIGS. 2, 3 and the related description at cols. 5 and 6, the electron beam with a spot size of 0.75 mm diameter is elongated into the electron beam with a spot size of 0.5 mm width and 4 mm length.
In this case, the cross section area of the electron beam with the spot size of the 0.75 mm diameter is 0.14 πmm2, and the cross section area of the electron beam with the spot size of the 0.5 mm width and the 4 mm length is 0.5 πmm2. As a result, the cross section area of the electron beam with the spot size of the 0.5 mm width and the 4 mm length is more than three times as large as the cross section area of the electron beam with the spot size of the 0.75 mm diameter. Therefore, the intensity of the thus obtained electron beam is decreased than the intensity of the original electron beam. In this point of view, in Reference 1, the intensity of the electron beam can not be increased even though the cross section of the electron beam is changed.
Moreover, in Reference 2, referring to FIGS. 4a and 4b and the related description of col. 5, the electron beam e is deflected out of the spiral plane over an extremely short distance in the Z-direction at the location of the radial field Br. In order to achieve such a deflection, the amplitude of the radial magnetic field is typically significantly larger than that of the axial magnetic field; for example, the axial magnetic field Bz may be 30 G, whereas the radial ejection field Br may be 110 G. At the exit from the Br field, the beam e′ focused in the φ-direction and steered onto the anode. However, Reference 2 does not refer to the increase of the intensity of the electron beam.
In Reference 2, FIG. 1 refers to the path of the electron beam in the beam guidance channel, but to the cross section of the electron beam.
[Reference 1] U.S. Pat. No. 6,181,771
[Reference 2] U.S. Pat. No. 5,680,432